The Labourer's Puzzle

Professor Rackbrane, during one of his rambles, chanced to come upon a man digging a deep hole. "Good morning," he said. "How deep is that hole?" "Guess," replied the labourer. "My height is exactly five feet ten inches." "How much deeper are you going?" said the professor. "I am going twice as deep," was the answer, "and then my head will be twice as far below ground as it is now above ground." Rackbrane now asks if you could tell how deep that hole would be when finished.